1. Field of the Invention
This invention relates to pulse repetition frequency detection systems and more particularly to a sequential processing system utilizing independent sorters for indicating the presence of pulse trains having predetermined pulse repetition frequencies.
2. State of the Prior Art
The detection and identification of a particular pulse train in a multiple pulse train environment may present difficulties, particularly in the field of electronic countermeasures (ECM) where countermeasures are to be employed against "enemy" radars tracking a target, e.g., by the transmission of jamming signals from the target. For instance, when pulsed radar systems are used to track a target, these radars may be jammed by the transmission of pulse trains having pulse repetition frequencies (PRF's) or rates at or near the PRF of the tracking radar. In such instances it is important for the tracked target to rapidly recognize the PRF of an incoming radar signal so that an appropriate signal can be generated. In a typical situation when a number of radars are tracking a target or are operating in the immediate vicinity, the composite signal received by the target may contain a large number of pulse trains at different PRF's and it is therefore desirable that the PRF's of the incoming signals be quickly determined so that signals from "enemy" radars can be recognized as such and appropriate countermeasures taken.
The detection of a pulse train having a particular PRF is particularly difficult in a multiple pulse train environment, i.e., with incoming signals having differing PRF's. Moreover, even if a signal having only one pulse train is detected, ascertaining the PRF thereof is difficult because of the wide range of possible PRF's encountered. The recognition of and the sorting out of the various pulse trains in a composite signal is also important in ascertaining the source of detected radar signals, i.e., to identify the signals as coming from "friendly" radars which may be safely ignored.
Sorting of the type described requires stable and rapid measurement to enable the employment of effective countermeasures. In all such systems an optimum balance must be maintained between detection probabilities and false alarm rates such that "friendly" pulse trains are eliminated from consideration, leaving a good probability of detection of "enemy" pulse trains.
Another problem with prior art ECM equipment is operator confusion due to "pulse summing" or the overlapping of pulses such that two or more pulses appear to the detection apparatus as a single pulse. Pulse summing may result in false or inaccurate indications of pulse train repetition frequency.
Additionally, there may be a problem of false alarms being generated at certain PRF harmonics of the pulse trains detected. When a large number of pulses are present, conventional parallel processing and masking techniques often fail to distinguish between the fundamental signal and lower harmonics. For instance, if the incoming signal is one in which 100 pulses per second are present, conventional processing equipment responding to 50 pulses per second may indicate the presence of a 50 pulse per second signal, thus giving a false indication of the PRF of the incoming signal.
Previously known systems for handling multiple pulse trains may include systems employing shift registers and/or special purpose computers. These systems may exhibit stability problems and thus result in high false alarm rates. Moreover, common clocking may result in cross-talk or other interference between the various elements of the detecting system reducing the system reliability.
In addition, prior art systems generally do not provide an easy method of locking out or inhibiting the indication of predetermined pulse trains such as those which are transmitted by "friendly" radars. Moreover, systems of the prior art do not generally provide for narrow band inhibiting which may be necessary when "friendly" and "enemy" radars are operating at nearly the same PRF's or at integrally related PRF's.
A further problem with generally known systems for detecting pulse trains having predetermined pulse repetition frequencies is the "look" time of the detecting apparatus, i.e., the amount of time required to detect a particular PRF. If a large number of pulses in the incoming composite signal are required for detection purposes, a significant amount of time may be lost in the processing of the composite signal and the detection apparatus may not be able to track the incoming signals. By rapidly varying the PRF of an "enemy" radar signal faster than these changes in PRF could be followed by the PRF detection apparatus, it is possible to counter efforts to jam an "enemy" signal.
The "look" time of the system may also be described as being the time during which the detection apparatus is active such that a real time output signal is obtainable. In known systems utilizing computer processing, a computer must search through its memory to ascertain the existence or nonexistence of a particular pulse train and the delay in searching the memory may be unacceptable under normal operating conditions.